This invention relates to the production of sintered uranium oxide containing compositions. One of the very important utilities of uranium oxide, especially uranium dioxide, is in nuclear power plants as a fuel in the generation of electric power. The uranium dioxide, either alone or in a mixture with other ceramics such as gadolinium oxide or plutonium oxide, is compacted to a given size and shape and sintered to achieve dense bodies for use in a nuclear fuel rod. The uranium present in uranium dioxide must be enriched with the U-235 isotope which is done in a gaseous state, the preferred practice being to use uranium hexafluoride. After enrichment it is necessary to convert the uranium hexafluoride to uranium dioxide. The resulting uranium dioxide can contain undesired fluoride ion concentrations and an oxygen to metal ratio above the desired ratio of about 1.98:1 to about 2.04:1.
The sintering of uranium dioxide structures has been used to attempt to reduce the oxygen and the fluoride content of the uranium dioxide. The current practice has been the use of wet hydrogen atmospheres at temperatures preferably greater than 1600.degree.C to achieve dense bodies of uranium dioxide. Past experience indicates a certain amount of water vapor with the hydrogen is required to remove the fluoride content from compacted ceramic structures during sintering, but the wet hydrogen process has not been satisfactory when the ceramic has high fluoride concentrations.
Another method presented in U.S. Pat. No. 3,375,306 for sintering dense uranium dioxide structures with or without ceramic additives is to heat the compressed powder at a temperature of 1300.degree.to 1600.degree.C in a sintering atmosphere of carbon dioxide or a mixture of carbon dioxide and carbon monoxide and cooling the sintered structure in a reducing atmosphere which varies as the composition of the structure varies. Where the structure being sintered is uranium dioxide the cooling gas is dry hydrogen, wet hydrogen or a mixture of carbon dioxide and carbon monoxide. Where the structure is uranium dioxide with an additive of plutonium dioxide, the cooling gas is steam or carbon dioxide mixed with carbon monoxide. The use of a mixture of carbon dioxide and carbon monoxide is more costly than use of wet hydrogen but enables the use of lower temperatures to achieve sintered structures of high density. However this carbon monoxide-carbon dioxide sintering atmosphere does not appreciably decrease the fluoride content of the uranium dioxide structures.
Sintering temperatures of about 1600.degree.C or higher produce uranium dioxide structures having large grain sizes with undesirable properties for some fuel applications. Uranium dioxide structures of smaller grain size have higher creep rates when compared to the creep rates of uranium dioxide structures of larger grain size. A higher creep rate for a uranium dioxide structure is desirable for better fuel performance. It has also been determined that other mechanical properties of finer grain size uranium dioxide are superior to the properties of coarser grain size uranium dioxide. The foregoing has made it desirable to lower the sintering temperature of uranium dioxides, and more generally of structures rich in uranium oxide, in addition to controlling the oxygen to metal ratio of the sintered structure and removing undesirable impurities from the sintered structure such as fluoride ions.
Lower sintering temperatures have other desirable features including cost savings as less power is expended in heating the sintering furnace, a longer functional life for the sintering furnace and its associated fixtures, less corrosion of the furnace components and the possibility of adapting a continuous conveyor belt or other means of transporting the structures rich in uranium oxide through the furnace.